Current-sensing circuits using operational amplifiers have been proposed as a technique for sensing electrical current from an inverter. In this current-sensing circuit, output voltage of the operational amplifier is input to a microcomputer, which calculates the current value from this output voltage. In such a current-sensing circuit, variations in the supply voltage of the microcomputer to which output voltage of the operational amplifier is input, LSB (Least Significant Bit) errors of the microcomputer, and resistance errors and the like cause errors in this current value. The lower the output voltage of the operational amplifier input to the microcomputer is, the larger such errors become.
Patent Document 1, for example, discloses a current-sensing circuit that employs an operational amplifier, in which the gain is variable based on the level of a current command value (so-called “reference voltage”) that controls the inverter. This current-sensing circuit sets a low gain when the current command value is large so as to be able to detect a high current flow, while it sets a high gain when the current command value is small so as to increase the current sensing resolution.
Patent Document 1: Japanese Patent Application Laid-open No. H10-132861
However, since the current from the inverter detected at the current-sensing circuit of the above-mentioned Patent Document 1 has a pulse wave form, the output voltage of the operational amplifier also has a pulse wave form. An expensive microcomputer with high arithmetic performance is necessary if such pulse-wave output voltage is to be input to the microcomputer to perform the arithmetic operation. Furthermore, since the current-sensing circuit of Patent Document 1 has a circuit configuration in which the current command value is input to a comparator and the gain is changed based on output from this comparator, the circuit requires additional circuit components such as the comparator, which leads to an increase in cost.